Electronic devices, particularly inverters for photovoltaic (PV) power systems have to fulfil a number of requirements. The desire for a high efficiency of such electronic devices results in high switching frequencies of power semiconductor switches which are used in the devices. These high switching frequencies in turn result in problems with electromagnetic compatibility (EMC) as the emission of electromagnetic oscillations has to be kept below predetermined threshold values. For this purpose, filter capacitors are employed on circuit boards of the electronic devices to dampen such oscillations. Some of these filter capacitors are connected to a circuit path with their first electrode and to a reference potential, like for example earth potential, with their second electrode. Often, the earth potential is provided on the circuit board in such a manner that one or more earth contact points on the circuit board are electrically connected to a metallic housing of the device via a metallic screw connection. The circuit board may also be mechanically fixed within the housing via these screw connections.
This method of contacting and fixing, however, often results in the formation of so-called dendrites, particularly with electronic devices which are used outdoors and which operate at high voltage differences on the circuit board. These voltage differences may both be DC voltage differences and AC voltage differences, particularly low frequency AC voltage differences. Dendrites are depositions which are, for example due to dirt, formed between circuit paths with a voltage difference. A few volts are sufficient for the formation of dendrites; the formation of dendrites being enhanced by higher voltage differences. Often, the dendrites have a sufficiently high electrical conductivity to cause an undesired electrical connection between neighboring circuit paths so that the electronic device does no longer function properly. In an extreme case, a dendrite may cause a short circuit which results in serious damage or even a destruction of the entire device.
Dendrites are primarily formed under the influence of deposits of moisture which may be formed in the interior of a housing of the device, like for example due to a condensation of air moisture. The high voltage values in operation of the device may result in an electrochemical transformation of the condensate or a reaction of the condensate with components of the circuit board, like for example corrosion, and thus enhance the growth of dendrites. Cold places in the housing are particularly prone to condensation and formation of dendrites. As the above mentioned screw connections between the circuit board and the housing form thermal bridges, they are typical places of condensation and formation of dendrites.
A further problem in manufacturing electronic devices results from the conditions of a so-called high voltage test which has to be passed, for example, by inverters for PV systems especially for security reasons. Such a high voltage test is, for example, defined in DIN EN 62109-1. During a high voltage test, an electronic device is subjected to voltages whose values are essentially above the voltage values which are reached in the allowed operation of the device. The high voltage test serves for ensuring the effectiveness of the electrical isolation measures of the device. In the high voltage test it is not essential to also test the filter capacitors for EMC, if their voltage resistance has previously been proven in a test carried out by their manufacturer. However, the filter capacitors are—as a rule—already fixed to the circuit board when the high voltage test is executed so that the full high test voltage drops over them. If the filter capacitors are failing in the high voltage test, like for example by explosion, this results in the necessity of a repair or an exchange of components or even in a destruction of the entire device. In case of an unnoticed failure of a filter capacitor, the EMC specifications may not be fulfilled by the electronic device.
Thus, it is the problem of the present disclosure to provide an electronic device which displays a reduced failure probability due to formation of dendrites even if used outdoors, wherein, at the same time, a simple and cost-saving manufacture of the device is possible. Further, a damage to components of the device, particularly to EMC filter capacitors, should be avoided during high voltage tests carried out.